Subject-verb agreement

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A colleague gave this explanation:

The choice of a plural or singular verb is determined by the form taken by the main subject of the sentence, which is not always necessarily the noun(s) closest to the verb in question. For instance, in the example you’ve provided the subject is whatever is being defined by “its/their”, and so a singular verb would be used with “its” and a plural with “their”. To provide a more practical example, with a sentence such as “The alloy’s microstructure and tensile strength was determined by…”, the singular verb “was” is used as it is the singular noun “alloy” that forms the main subject of this sentence (microstructure and tensile strength simply being singular characteristics of this subject”.

2 answers

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I disagree with your colleague. The subject of this sentence is not "alloy." It is the compound subject "microstructure and tensile strength." A possessive noun is used as an adjective to modify another noun.

Look at this sentence:Joe's car was expensive.

The subject of the sentence is not Joe. It is car. Joe is modifying car. It could be Joe's big, red, fast car is expensive. Everything before car are modifiers of the noun. In your example sentence, alloy's is modifying both parts of the compound subject.

When a compound subject is connected by and, even when both nouns are singular, that makes a plural subject. The plural verb is used.

There are occasions when a compound subject with and is used for a singular item. In that case, the singular verb is used.

Peanut butter and jelly is my favorite kind of sandwich.

The peanut butter and jelly are in different aisles at my grocery store.

When two singular nouns (or pronouns) are joined with or, then the compound subject is singular. When or is used with a singular noun and a plural noun, the verb agrees with the part of the compound subject that is closest to it. Always.

It is true that there may be a noun (not the subject) that is closer to the verb. Instead of a possessive noun, you could use a prepositional phrase after the subject:

The microstructure and tensile strength of the alloy were determined by . . .

Whether you modify the subject with a possessive noun or a prepositional phrase, it is a plural compound subject that needs a plural verb to agree.

And if the tensile strength is also determined by that same thing, but independently,

Then the answer is "was".

If both of those characteristics are determined by the same thing and then change at the same time (proportional), I think you would rather use "were". It would be taken as a whole and the two wouldn't be considered as two singuliar parts, but one plural part.